The present invention relates to a coupling mechanism for coupling a signal current including a high-frequency signal component and a low-frequency signal component between two different potentials.
A consequence of the progression to shorter channel lengths and thinner gate oxides in MOS technology is that supply voltages become progressively lower. This is due primarily to lower breakdown voltage of thinner gate oxides. In analog signal processing circuits, lower supply voltages present a number of serious difficulties.
One such difficulty, sometimes referred to as the xe2x80x9cstacking problemxe2x80x9d, can be understood by considering the example of a Gilbert-Cell multiplier 10, shown in FIG. 1. In the Gilbert-Cell multiplier 10, a differential input voltage Vin is converted to a differential current ia using a transconductance stage comprising transistors 11 and 12, resistors 14 and 16, and current source 18. The relationship between ia and Vin is approximately
ia=gm Vin
The current ia is then steered using transistors 20, 22, 24 and 26 in such a way that a fraction k of the current goes through the left path and a fraction l-k goes through the right path. This produces an output current io which is proportional to the product of Vin and k,
io=gm k Vin
thus performing a multiplication operation.
The problem is that because the transistors are xe2x80x9cstackedxe2x80x9d in such a way that the same current signal flows through both a bottom transistor (11-12) and a top transistor (20, 22, 24 and 26), the supply voltage must be large enough to accommodate the collector-emitter drops for both transistors. The problem can be solved in some cases by xe2x80x9cunstackingxe2x80x9d the Gilbert-Cell into two separate stages and capacitively coupling the two stages together, as shown in FIG. 2. Capacitors 50 and 52 provide DC isolation, allowing the common mode voltage to be different in the two stages, while also providing AC connectivity, passing the signal current between the stages. Current sources 46 and 48 provide a DC path for the current generated in the transconductance stage.
The primary problem with the capacitive coupling method of xe2x80x9cunstackingxe2x80x9d is that the coupling capacitor limits the band of frequencies the circuit can accommodate. The circuit no longer passes DC signals at all, and low frequency signals are highly attenuated. For many applications, such as radio, this is not a significant problem, because the signals do not extend all the way down to DC. In the case where the signal represents baseband data, for example in optical networking, the inability to pass DC and low frequency signals is a very significant problem, however.
In some cases, the current sources 48 and 46 can be implemented with resistors. This is because the input to the second stage has a low impedance, so that the voltage swing at the output of the first stage is small.
In some cases, the capacitive coupling method requires a common-mode control circuit (not shown in FIG. 2) to set the DC common mode voltage level of the first stage.
Thus, there exists a need in analog signal processing for a coupling mechanism between two different potentials that transmits DC signals as well as AC signals between the two different potentials even in a situation where the supply voltage is low.
In one embodiment, the invention is an apparatus for coupling a wideband current signal between two different potentials. The apparatus incorporates a capacitor for providing a signal path for a high frequency signal from a first potential to a second potential. The apparatus further incorporates a current mirror for providing a signal path to a low frequency signal from the first potential to the second potential.
In an alternate embodiment, the invention is a method for coupling a signal current between a first potential and a second potential. The method includes receiving a high frequency signal from the first potential. The method also includes receiving a low frequency signal from the first potential. The method further includes transmitting the high frequency signal via a high frequency signal path to the second potential. Moreover, the method includes transmitting the low frequency signal via a low frequency signal path to the second potential.